With the evolving of semiconductor technologies, semiconductor chips/dies are becoming increasingly smaller. In the meantime, more functions need to be integrated into the semiconductor dies. Accordingly, the semiconductor dies need to have increasingly greater numbers of I/O pads packed into smaller areas, and the density of the I/O pads rises quickly over time. As a result, the packaging of the semiconductor dies becomes more difficult, which adversely affects the yield of the packaging.
Conventional package technologies can be divided into two categories. In the first category, dies on a wafer are packaged before they are sawed. This packaging technology has some advantageous features, such as a greater throughput and a lower cost. Further, less underfill or molding compound is needed. This packaging technology suffers from drawbacks. For example, the sizes of the dies are becoming increasingly smaller, and the respective packages can only be fan-in type packages, in which the I/O pads of each die are limited to the region directly over the surface of the respective die. With the limited areas of the dies, however, the number of the I/O pads is limited due to the limitation of the pitch of the I/O pads. If the pitch of the pads is to be decreased, solder regions may bridge with each other, causing circuit failure. Additionally, under the fixed ball-size requirement, solder balls must have a certain size, which in turn limits the number of solder balls that can be packed on the surface of a die. Accordingly, Integrated Fan-Out (InFO) packages have been developed.
InFO packages are not suitable for making coils that are used for certain applications such as wireless charging. Due to the small size of the InFO packages, the coils, if made in the InFO packages, would be small. The mutual inductance between the coils in the InFO packages and the coils outside of the InFO packages will be low, and cannot meet the requirement of wireless power transfer through magnetic resonance. On the other hand, the mutual inductance also cannot be increased by increasing the number of turns of the coils since this will cause the resistance to be increased, which in turn causes the dramatic reduction of the efficiency of the power transfer.